The present invention relates to a common mode choke coil adapted for surface mount applications to remove common mode noises and, particularly, to a closed magnetic circuit structure of a common mode choke coil having external electrodes joined directly to a flange of a core in order to have a higher bonding coefficient.
For realizing a high-density surface mount package on a printed circuit board by using a chip mounter (a chip mounting apparatus), various electronic components including resistors and capacitors have been minimized in size or thickness and/or formed into a chip form. Common mode choke coils of a winding type, for attenuating noises on DC lines and unwanted radiation on secondary lines of a source or signal lines, are also provided for surface mount applications. One such choke coil is disclosed in Japanese Patent Application Laid-Open No. 8-186028 entitled xe2x80x9cWinding Chip Inductor With Gapxe2x80x9d, wherein a common mode choke coil allows external electrodes to be joined directly to a flange of its open magnetic circuit type core, hence forming an almost-closed magnetic circuit structure.
FIGS. 6(a)-6(c) illustrates such a conventional common mode choke coil 10 wherein a core 1 of an open magnetic circuit type has a couple of flanges 3, 3 of a cross figure in cross section provided integrally on both sides a column-like winding core 2 thereof, each cross-figured flange having a lower raised portion thereof coated with a silver paste and joined directly by thermal bonding with a pair of external electrodes 5a and 5c or 5b and 5d, in which both external electrodes are further electrically connected by soldering or the like to two leads 6 and 7, respectively, which extend from the winding core 2 of the open magnetic circuit type core 1. In addition, the open magnetic circuit type core 1 is joined at the top to an I-shaped core 9 (a rectangular plate) by gap providing mediums 8.
The core 1 in the above structure is a ferrite core, which has a high resistance and can be joined directly to the external electrodes 5a to 5d. 
Since the above conventional common mode choke coil 10 is not covered with resin molding as an external packaging, it can be small in size. However, tape material, liquid adhesive or a combination thereof is used as a gap forming medium 8 between the open magnetic circuit type core 1 and the I-shaped core 9. Accordingly, the gap is large, as apparent from FIG. 6(b), causing sufficient magnetic flux leakage to occur to cause a decline in the bonding coefficient.
Also, winding wires of the winding core are exposed but not protected with any resin molding. Thus, an electrical short-circuit or a break-down is quite likely to occur when the choke coil 10 is being handled, hence affecting the reliability of the component.
It is an object of the present invention, in view of the foregoing aspects, to provide a smaller, more reliable common mode choke coil suitable for surface mount type chip applications incorporating a closed magnetic circuit in which the structure allows for a higher bonding coefficient.
To achieve the object of the present invention, there are provided five embodiments as follows:
In the first embodiment, a common mode choke coil is provided comprising a ferrite core having two substantially square planar flanges formed integrally on both ends of a winding core thereof, each flange joined directly with a pair of external electrodes a plurality of windings wound about the winding core of the ferrite core and connected by thermal pressure bonding to sides of the external electrodes at recesses provided in the left and right sides of each flange, and a ferrite plate bonded by an adhesive to the outermost surface of the windings to join the two flanges at both ends of the ferrite core;
In the second embodiment, a common mode choke coil is provided comprising a ferrite core having two substantially square planar flanges formed integrally on both ends of a winding core thereof, each flange joined directly with a pair of external electrodes and having beveled faces provided on the winding core side thereof facing the opposing flange, a plurality of windings wound about the winding core of the ferrite core and connected by thermal pressure bonding to sides of the external electrodes at recesses provided in the left and right sides of each flange, and a ferrite plate bonded by an adhesive to the outermost surface of the windings and tightly fitted against the beveled faces of the flanges to securely join the two flanges at both ends of the ferrite core.
In a third embodiment, a common mode choke coil is provided according to the first and second embodiments above, wherein two of the ferrite plates are attached to both left and right sides of each flange to join the two flanges at both ends of the ferrite core.
In a fourth embodiment, a common mode choke coil is provided according to embodiments one through three above, wherein the windings wound about the winding core of the ferrite core are thoroughly covered with a resin coating material, and the ferrite plate(s) are bonded to the resin packaging material to join the two flanges at both ends of the ferrite core; and
In a fifth embodiment, a common mode choke coil is provided according to the fourth embodiment above, wherein each flange of the ferrite core has relief grooves provided in the sides thereof for allowing an excess of the resin coating material to flow out during the coating process.
In each of the coil structures according to the first through fifth embodiments of the present invention described above, the ferrite plate(s) joining the two flanges is electrically connected to the flanges at both ends of the ferrite core, thus yielding a closed magnetic circuit together with the winding core. Accordingly, the resultant common mode choke coil of the present invention has a higher bonding coefficient.
The coil structure of the second embodiment has an advantage over prior art structures in that the ferrite plate, at its ends, is tightly fitted against the beveled faces (slopes) of the two flanges at both ends of the ferrite core, thus allowing for smaller overall size and yielding a closed magnetic circuit. In the coil structure of the third embodiment, the ferrite plates are joined to the two, left and right, sides of the ferrite core, thus minimizing the overall height of the structure. In the coil structure of the fourth embodiment the windings are thoroughly covered with the resin packaging/coating material to develop a chip construction, thus allowing for the coil structure to be handled with ease. The coil structure called for in the fifth embodiment facilitates the packaging/coating process for covering the windings with the resin packaging/coating material.